Nuclear magnetic resonance (NMR=Nuclear Magnetic Resonance) is a phenomenon possessed by some atom nuclei and quite extensively used e.g. in the study of solid matter physics, chemistry and biology. The latest application of NMR is NMR imaging, the distribution of NMR parameters in an object to be examined (e.g. a human body) being presented as a map that can be used e.g. in medicine.
In NMR study, an rf-(=radio frequency) pulse is transmitted to a target in a static magnetic field, the frequency of said pulse being close to the so-called resonance frequency of the nucleus to be examined and its direction being substantially orthogonal relative to said static magnetic field. This pulse excites the desired nuclei making them precess around said static magnetic field. This precession induces in a receiver coil an NMR signal from which the desired parameters can be calculated.
An NMR examination apparatus is always provided with transmitter and receiver coils that can be separate or one single coil. In NMR imaging, the target to be examined is usually placed inside the receiver coil in an area where the rf field is as homogeneous as possible in order to accentuate all sections in the same way and to produce a reliable image.
However, the object of interest is often a small section of a larger target. Typical examples include the ophthalmic or otologic area in a human body, the outer layers of a body when measuring chemical shifts etc. Thus, it preferable to use a receiver coil which is sensitive to a signal originating from this particular section. Since the inducing signal is inversely proportional to the distance from the conductors of a receiver coil, this is accomplished by means of a small receiver coil that can be brought close to a target to be examined. This prior art solution, a so-called surface coil, can be used either as both transmitter and receiver or as a receiver only, in which case the excitation is effected by means of a larger transmitter coil producing a homogeneous rf-field.
A surface coil is most commonly used in an NMR imaging system intended for imaging also the entire body (or head). In this case, the introduction of a surface coil requires at least the following actions: (1) a surface coil must be connected to the receiving electronics of a system, (2) if the transmitter coil used is the transmitter/receiver coil of the actual system, the coupling thereof to the surface coil must be prevented for the signal detection period, (3) if the surface coil also serves as a transmitter, it must be connected to the transmitting electronics of the actual system (also in this case), the coupling to the transmitter/receiver coil of the actual system must be prevented).
The connection of a surface coil to the receiving electronics is generally effected in a manner that the surface coil is fitted with its own preamplifier and a signal is applied to the actual system at a later stage in the amplifier/mixer sequence. In this connection, the optimization of the noise characteristics of an apparatus demands rather high accuracy: blocking out external disturbances in quite crucial in NMR imaging. The transmitter and receiver coil arrangements nearly always involve the use of timed circuits. In the above-described case, between the surface coil and the transmitter/receiver coil of the actual system there is established a coupling which disturbs testing. In order to prevent the coupling, the surface coil can be set to be orthogonal relative to the actual coil (sensitive to movements), or the actual coil can be provided with a connection for cutting off the circuit for a signal receiving period.